This invention relates to a control system for a reciprocating linear motor and, more particularly, to a high efficiency tri-state switching controller for a reciprocating linear motor.
In certain industrial applications linear motors have been found to have important advantages over the more conventional rotary motor. One such application is in the compressors of cryogenic refrigerators where the extremely low temperatures (20.degree.-60.degree. K.) cause condensation of any impurities in the helium working gas. In order to ensure the purity of the helium, the use of lubricants is prohibited which severely reduces the life of conventional apparatus necessary to produce linear motion. This is particularly true in the case where cryogenic refrigerators are used in a remote and inaccessible area, such as outer space.
It is customary to produce linear motion by means of a conventional rotary motor in conjunction with a mechanical to linear converter. The mechanical parts of these devices are subject to wear and thus limit the useful life of the apparatus. In addition, the use of such devices in environmental extremes makes it difficult for the motor seals and other critical elements to function properly, thereby causing equipment failure. Linear motors were therefore developed to alleviate the problems inherent in conventional rotary to linear conversion devices. For background, see U.S. Pat. No. 4,389,849 (6/28/83).
Axial control of reciprocating linear motors is typically accomplished utilizing conventional servo loop control techniques which employ an LVDT (Linear Variable Displacement Transducer) axial position sensor, a PID (Proportional-Integral-Derivative) controller and a high frequency switching amplifier. A 70% efficient 100 KHz switching amplifier has been used in a 75 Hz bandwidth classical control loop to sinusoidally drive a 400 watt piston compressor at 25 Hz. The relatively low bandwidth of the control system (limited by the bandwidth of the position sensor and the motor) resulted in a steady state amplitude control that was only marginally adequate and a less than adequate phase control at 25 Hz. The system reliability and efficiency was inadequate for use in a long life Stirling cycle cooler to be used in space missions.